1. Field of the Invention
This invention relates generally to oxygen generating compositions, and more particularly concerns improved oxygen generation compositions containing magnesium oxide as an additive to modify and smooth the oxygen generation, suppress chlorine formation, and to facilitate mixing of the compositions.
2. Description of Related Art
Chemical oxygen generating compositions based upon the decomposition of alkali metal chlorates or perchlorates have long been used as an emergency source of breathable oxygen, such as in passenger aircraft, for example. Oxygen generating compositions utilizing alkali metal chlorates or perchlorates are disclosed in U.S. Pat. Nos. 5,198,147; 5,279,761; and 5,298,187; each of which are incorporated herein by reference.
A typical chemical oxygen generating candle may have several layers with different compositions and thus different reaction rates. Multiple layers are used in the candle instead of a single formula to help match the oxygen generation requirements, which vary with time as an airplane descends following a loss of cabin pressure. Different applications have different oxygen generation requirements. The candle typically has a generally cylindrical shape with a taper, with a recess at one end to hold an ignition pellet. The ignition pellet is typically ignited by firing a primer, and heat from the ignition pellet then ignites the reaction of the candle body, generating oxygen.
An aircraft oxygen generator is commonly preprogrammed according to the descent profile of a given type of airplane, and must meet the minimum oxygen flow requirement at all times during a descent. However, oxygen gas formed inside a chemical oxygen generation core or candle must develop sufficient pressure to escape from the core. This effect can cause an uneven and erratic smooth flow of oxygen from an activated oxygen generation core. In order to meet minimum flow requirements despite such variations in oxygen flow from the core, excess weight of the chemical oxygen generating composition is commonly used. To minimize unnecessary weight, particularly in aircraft, it would be desirable to provide oxygen generating compositions that can facilitate the smooth flow of oxygen from an activated oxygen generation core.
Barium peroxide, lithium peroxide and calcium hydroxide have been used together with cobalt oxide as inhibitors to moderate the performance of sodium chlorate in oxygen generating compositions. However, barium peroxide is toxic, so that disposal of expended and scrap oxygen generators containing barium peroxide can be expensive. Lithium peroxide and calcium hydroxide are very strong inhibitors, so that only small quantities, such as a fraction of one percent in the oxygen generating compositions, for example, can be used, making it relatively difficult to uniformly distribute the inhibitors in the oxygen generating compositions. Lithium peroxide also tends to cake when it adsorbs moisture and carbon dioxide, making mixing even more difficult. For a formulation containing cobalt oxide as a catalyst and lithium peroxide or calcium hydroxide as an additive, a prolonged and thorough mixing is critical to reduce the variation within each lot, and from one lot to the next. Decomposition of calcium hydroxide also produces water which can be undesirable for some applications. Calcium hydroxide also has minimal catalytic activity when no other catalyst is present.
Since calcium hydroxide is a relatively strong inhibitor, in oxygen generating compositions formulated with cobalt oxide and calcium hydroxide, localized regions having a high cobalt oxide concentration and a low calcium hydroxide concentration occur due to imperfect mixing, and have a far higher decomposition rate than other localized regions with a low cobalt oxide and a high calcium hydroxide concentration. Lithium hydroxide and calcium hydroxide based formulations of oxygen generating compositions thus typically exhibit a relatively high variation of performance within and among lots, due to nonuniform distribution of the ingredients within the oxygen generating compositions. It would be desirable to provide an improved oxygen generating formulation in which a uniform distribution of ingredients is easier to achieve, which will produce a greater uniform performance, and which is easier to manufacture than conventional formulations. The present invention meets these needs.